This invention relates to a tuning mechanism for superconducting radio frequency (SRF) particle accelerator cavities. Tuning the frequency of a cavity takes place when the cavity is stretched or compressed along its beam axis, thereby changing its geometry and thereby its resonant frequency.
SRF particle accelerator cavities need to be tuned in order to have maximum efficiency. A tuner at Jefferson Laboratories consists of a lead screw motor, two cell holders, and a dead leg. The cell holders are on each of the outer most cells. The lead screw and dead leg are connected to the cell holders on opposite sides of the cavity. One cell holder is rigid and the other is two parts, with an outer disk that pivots around the cell holder as the motor moves the disk. The pivot axis is perpendicular to the lead screw and dead leg, and is connected to the cell holder. As the motor progresses it rotates the disk, thereby pulling the outer cells apart. This stretches the cavity, which changes its resonant frequency.
A nickel magnetostrictive tuner system has been used for fine tuning these cavities. This system consists of a solid nickel rod that replaced the dead leg of the lead screw tuner described above. A superconducting coil surrounding the rod is used to activate the fine tuner. This system, however, requires a long rod of nickel since the magnetostriction of nickel is only about 30 ppm. The longer rod also requires a larger solenoid, creating a larger magnetic shielding problem for the SRF cavity system. The cavities are very sensitive to the presence of magnetic field during the cool down through the superconducting transition temperature.
Piezoelectric tuners could be used, however they do not operate at cryogenic temperatures, have low force output, and require high operating voltages. Having to feed a motor through the vacuum insulation causes a temperature gradient from the helium vessel to room temperature resulting in a larger heat load for the refrigeration system. Also, the low force output of piezoelectric motors requires the system to have a separate high force motor to do the coarse tuning. The voltage requirements for running a piezoelectric motor are five hundred to a few thousand volts.
CERN uses an SRF tuner with a room temperature motor that feeds through the cryostat to a lever system. The motor pulls ropes that twist rectangular bars on either side of the cavity. The bars have metal foils that connect the bars to the cavity and a rigid frame. As the bars are twisted, the foils rotate and pull the cavity and the frame together. The major disadvantage of this system is that the motor is located outside the cold source. This creates a temperature gradient across the feed through, warming the inside of the cryostat.
The APT tuner designed at LANL is composed of a motor that pushes on a lever arm. The lever arm is attached to plates on both sides of the neck of the cavity. Each plate has an intricate design of cuts to ensure lateral motion. Because of the time and detail that must go into the machining of these components, the tuner is very expensive.
Up to this point, the prior art tuning of particle accelerator cavities has been a choice of poor precision at low temperatures, or high precision while using a tuner outside of the cold source. SRF tuners up to this point have been very expensive mechanisms to build.
One difference between the inventive tuning mechanism and the prior art is the application of magnetic smart materials for motion. Magnetic smart materials change shape upon the application of a magnetic field. Elongating the material axially causes desired motion. Prior art piezoelectric materials rely on high voltages in order to elongate. Prior art lead screws are purely mechanical devices. Preferred materials comprise TbDyZn or TbDyFe alloys, which have strains of up to 5000 ppm. Such materials are disclosed in U.S. patent application Ser. No. 09/970,269, incorporated herein by reference.
The lever arm in the inventive motor also uses a higher mechanical advantage than other tuners, requiring less force from the motor and increasing the realized precision on the cavity from the motion of the motor. Wire ropes attached to the lever deal with axial loading; the wire ropes are pivoted at one end, allowing the transverse displacement of the lever to be negligible in the tuning of the cavity.
The inventive tuner combines high force and high precision at cryogenic temperatures. Another major advantage of the inventive tuner is its simplicity. Its low number of uncomplicated parts makes the tuner inexpensive to build and easy to setup and control. In addition to cost, magnetic smart materials require voltages 500 times less than those of piezoelectric materials to operate.
Precision Positioning:
The inventive motor can position with sub-micron precision. The lever arm has a mechanical advantage that also serves to increase precision. For every given amount the motor positions, the cavity is stretched a fraction of that displacement.
Elimination of Mechanical Feed Throughs in Cryostat:
Mechanical feed throughs cause heat to be leaked into the cryostat. A vital aspect of superconductivity is the ability to maintain low temperatures. The inventive motor can be entirely enclosed in the cold source. Only the coil leads have to be fed through the cryostat. There are commercially available feed throughs to translate an electrical signal through a cryostat without leaking any heat through the vacuum vessel.
Low Magnetic Fields:
Magnetic smart materials can achieve saturation of 5000 ppm at very low magnetic fields. These magnetic fields are about 1500 Oersteds, making it very easy to block the magnetic field from affecting the operation of the accelerator.
Low Voltage Operation:
The inventive tuner uses superconducting coils to produce the magnetic field at cryogenic temperatures. The superconducting coils carry high currents, approximately 5 to 10 amps, but require less than 2 volts to operate.
Low Temperature Operation:
Most particle accelerators currently being built or designed are superconducting. They have operating temperatures of below 4K, which suits well to the inventive tuner. The low temperatures allow the tuner to utilize superconducting coils, which can supply the magnetic field with negligible resistance in the coils. Therefore there is negligible heat dissipation and low voltage requirements.
No Lubricants Needed:
A major problem that engineers face when designing motors for cryogenic applications is the absence of lubrication. There are no lubricants that can survive cryogenic temperatures. Any motor with moving parts is going to require lubrication to offset wear. Utilizing magnetic smart materials to provide motion eliminates the need for lubrication.
This invention features a tuning mechanism for a superconducting radio frequency particle accelerator cavity, wherein the cavity comprises a number of axially aligned cells held by a frame, with at least one active cell that is axially stretchable to tune the resonant frequency of the cavity, the tuning mechanism comprising: a lever arm having a center of rotation; one or more mechanical members coupling the lever arm to an active cell; and a motor adapted to move the lever arm, to thereby move the active cell through the mechanical members.
The frame may comprise an end member spaced from the active cell. The lever arm may be located at least in part between the frame end member and the active cell. There may be a plurality of mechanical members coupling the lever arm to the active cell. The coupling from the lever arm to the active cell may be indirect. The tuning mechanism may further comprise an active cell holder coupled to the active cell. The plurality of mechanical members may connect the lever arm to the active cell holder.
The mechanical members may be coupled to the lever arm on one side of the center of rotation of the lever arm. The tuning mechanism may further comprise one or more additional mechanical members coupling the lever arm to the frame. The additional mechanical members may be coupled to the end member of the frame. The additional mechanical members may be coupled to the lever arm on the other side of the center of rotation of the lever arm.
The additional mechanical members may comprise wire ropes. The tuning mechanism may further comprise a guide over which each wire rope runs between the lever arm and the frame end member to change the direction of the wire rope to translate the direction of force on the frame end member from the lever arm. The mechanical members may comprise wire ropes. The tuning mechanism may further comprise a guide over which each wire rope runs between the lever arm and the active cell, to change the direction of the wire rope to translate the direction of motion of the lever arm to a different direction of motion of the active cell.
The motor may comprise a translating member that pushes on the lever arm, comprising a material that is elongated upon application of a magnetic field, and the motor may further comprise a coil proximate the material for providing a variable-strength magnetic field to the material. The material may be magnetostrictive. The motor may further comprise means for selectively clamping the translating member to inhibit its motion. The tuning mechanism and cavity may be operated at below 4 degrees Kelvin.
The motor may be rigidly connected to the frame. The frame may further comprise an inactive cell holder coupled to the cell furthest from the active cell, and a series of rigid frame rods connecting the inactive cell holder to the frame end member. The motor may comprise a rotating lead screw that pushes on the lever arm.
The tuning mechanism may further comprise a liquefied gas containing vessel surrounding the cavity, the vessel defining a flexible bellows, and the lever arm may be coupled to the vessel across the bellows with coupling on one side of the bellows to the lever arm on one side of the center of rotation, and coupling on the other side of the bellows to the lever arm on the other side of the center of rotation.